By Alan Baldwin

LONDON (Reuters) - Buckingham Palace will provide a royal backdrop to the London 2012 Olympic cycling road race with riders starting and sprinting to the finish outside Queen Elizabeth's residence, organizers said on Thursday.

The men and women's road races are provisionally scheduled for the first two days of the Games on July 28 and 29.

The race route will run south-west from The Mall outside the palace in central London and past landmarks in the boroughs of Westminster, Kensington and Chelsea, Hammersmith and Fulham to cross the River Thames at Putney Bridge.

From Dorking the race repeats a 15.5km circuit around Box Hill, a loop that will be repeated several times.

"This extends the Games into the south west of London and Surrey," said London organizing committee chairman Sebastian Coe in a statement.

"We are delighted to be bringing such high profile and exciting events here, and which will be bringing the road race into challenging cycling terrain through many new boroughs and districts."

Mark Cavendish, one of the big home hopes and winner of five stages on last year's Tour de France, looked forward to the challenge.

"Competing in a home Olympic Games is a once in a lifetime opportunity. To compete in the road race on the opening weekend of the London 2012 Games in front of home fans is going to be amazing," he said.

The men's race, won in 2008 by Spain's Samuel Sanchez, will take around six hours with 145 riders covering 250km.

Briton Nicole Cooke won the women's race in Beijing, and the 2012 event measures 140km with 67 riders.

"I think this route will produce an extremely exciting Olympic Games road race, and especially with the circuits of Box Hill it will be a worthy winner of both the men's and women's gold medals," said Pat McQuaid, president of the International Cycling Federation (UCI).

Organizers said a test event will be held along the same route in August this year.

(Editing by Tom Pilcher)